1. Field of the Invention
The present invention generally relates to a pressure sensor for measuring a pressure applied on a diaphragm thereof, and more particularly to a pressure sensor for detecting a combustion pressure of a combustion gas changing in a combustion chamber of an internal combustion engine.
2. Description of Related Art
A pressure sensor has been mounted on a vehicle to measure a combustion pressure of a combustion gas in a combustion chamber of an internal combustion engine. This sensor has a case, a pressure receiving diaphragm fixed to the case on one end side of the case, a pressure transmitting member attached to the diaphragm in the case, and a pressure detecting unit disposed on the other end side of the case. The detecting unit has a bridge circuit composed of a plurality of deformation gauges. The diaphragm has a pressure receiving surface exposed to the gas of the chamber. When a combustion pressure of the gas is applied to the surface of the diaphragm, the diaphragm is deformed and adds a load corresponding to the pressure to the pressure transmitting member. The member transmits this load to the pressure detecting unit, and the unit converts the load into an electric signal indicating a level of the pressure. Therefore, the sensor can detect the combustion pressure.
In this sensor, the gas reaches a high temperature such as 1000° C. or more when being exploded in the chamber, so that a thermal stress is caused in the diaphragm due to the heat of the gas. The diaphragm is deformed due to this stress so as to add a load to the pressure transmitting member. Therefore, the diaphragm cannot correctly add a load corresponding to the pressure to the member, so that the sensor cannot correctly detect the pressure. To solve this problem, Published Japanese Patent First Publication No. 2004-347387 has disclosed a pressure sensor wherein a diaphragm has a pressure receiving surface formed in a specific shape so as to reduce adverse influence of the heat of the gas on the diaphragm. Therefore, deformation of the surface caused by the heat of the gas can be reduced in some degree.
However, an engine speed and/or a required engine torque or load frequently vary with time, so that combustion conditions of the gas such as a combustion pressure, a combustion temperature and the like are frequently changed. In this case, influence of the thermal stress on the diaphragm is variously changed in response to the combustion conditions. When the combustion conditions are variously or frequently changed, the sensor merely formed in a specific shape cannot correctly detect the pressure.
Further, U.S. Pat. No. 5,471,883 has disclosed a pressure sensor wherein a cooling system is disposed to cool an inner surface of the diaphragm with water. In this sensor, because the temperature of the diaphragm is almost adjusted within a predetermined range, the influence of the thermal stress on the diaphragm can be considerably reduced. However, because the cooling system is additionally disposed in the sensor, a size of the sensor becomes large, and a manufacturing cost of the sensor becomes heightened.